From mboxrd@z Thu Jan  1 00:00:00 1970
From: Dario Faggioli <dfaggioli@suse.com>
Subject: Re: Ongoing/future speculative mitigation work
Date: Thu, 25 Oct 2018 18:01:50 +0200
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To: Tamas K Lengyel <tamas.k.lengyel@gmail.com>, Andrew Cooper <andrew.cooper3@citrix.com>
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On Wed, 2018-10-24 at 09:24 -0600, Tamas K Lengyel wrote:
> > A solution to this issue was proposed, whereby Xen synchronises
> > siblings
> > on vmexit/entry, so we are never executing code in two different
> > privilege levels.  Getting this working would make it safe to
> > continue
> > using hyperthreading even in the presence of L1TF.  Obviously, its
> > going
> > to come in perf hit, but compared to disabling hyperthreading, all
> > its
> > got to do is beat a 60% perf hit to make it the preferable option
> > for
> > making your system L1TF-proof.
>=20
> Could you shed some light what tests were done where that 60%
> performance hit was observed?=20
>
I don't have any data handy right now, but I have certainly seen
hyperthreading being beneficial for performance in more than a few
benchmarks and workloads. How much so, this indeed varies *a lot* both
with the platform and with the workload itself.

That being said, I agree it would be good to have as much data as
possible. I'll try to do something about that.

> We have performed intensive stress-tests
> to confirm this but according to our findings turning off
> hyper-threading is actually improving performance on all machines we
> tested thus far.
>=20
Which is indeed very interesting. But, as we're discussing in the other
thread, I would, in your case, do some more measurements, varying the
configuration of the system, in order to be absolutely sure you are not
hitting some bug or anomaly.

Regards,
Dario
--=20
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Software Engineer @ SUSE https://www.suse.com/

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